The invention relates to a system that simulates the braking feel at the brake pedal. It applies more particularly to a vehicle fitted with a conventional braking system in which the brake pedal acts on the hydraulic braking circuit that brakes the wheels of the vehicle, and with an auxiliary energy braking system such as an electric braking system. It applies notably to hybrid vehicles such as vehicles with combined traction comprising an internal combustion engine and one or more electric motors.
In the state of the art, it is known practice to fit out vehicles using two braking systems operated by the same operating device (brake pedal), and acting on the wheels of the vehicle using different sources of energy or power.
This is the case, for example, of hybrid vehicles with combined traction that have an internal combustion engine, for traction, and electric traction motors. In these vehicles, it is known practice to use the electric motors for braking, by operating them as electric generators, braking then being had by recuperation of energy.
The vehicle therefore comprises an energy recuperation electric braking system but it generally also comprises a conventional hydraulic braking system because the braking force for the electric braking system is limited and becomes very low at low speed.
However, electric braking is not always entirely satisfactory. FIG. 1 schematically depicts, in continuous line, a braking curve (braking torque as a function of time) for one such electric braking system alone. In this same graph, a dotted line has been used to represent vehicle speed. In a first braking zone A, the braking increases gradually until it reaches its full effectiveness in a second braking zone B, then as the vehicle speed becomes low, in a third zone C, the braking torque decreases, becoming practically non-existent in a fourth zone D. A braking system such as this is imperfect because in zone A the braking does not reach full effectiveness quickly and because in zones C and D the braking decreases when the vehicle is at low speed. This kind of operation is depicted, in FIG. 1, by the curve in dotted line.
To remedy this disadvantage with recuperative electric braking, it is necessary to provide a hydraulic braking system which provides supplemental braking in zones A, C and D.
FIG. 2 shows curves of the operation of the electric braking system and of the hydraulic braking system.
Curve C1 is the curve of recuperative electric braking that corresponds to that of FIG. 1. If a vehicle deceleration of, for example, 0.7 g is desired, and the electric braking system is able to obtain a maximum deceleration of 0.3 g, then the braking curve applied by the hydraulic braking system will need to be the one schematically depicted by curve C2. In zone A, the hydraulic braking torque will as soon as possible allow a deceleration of 0.7 g and will then decrease until the electric braking torque reaches its maximum value and the equivalent deceleration due to this electric braking torque is 0.3 g. In this way, at any moment, the sum of the torques supplied by the electric and hydraulic braking systems provides a deceleration of 0.7 g. In zone B, the hydraulic braking system supplements the electric braking system in order also to obtain a deceleration of 0.7 g. Thereafter, in zone C, there is an increase in the braking torque of the hydraulic system in order once again to supplement the braking of the electric braking system and, in zone D1, to compensate for the practically non-existent electric braking torque.
Moreover, leaving aside the response times of the recuperative electric braking system, this system may not respond the same way all the time because the load on the recuperative circuit may vary. This is particularly the case when the recuperative circuit essentially comprises the vehicle batteries because in this case the load can vary according to the state of the batteries.
In such systems, a control circuit (for example a computer) has to manage the operations of the braking systems. This computer is called into service for each braking action.
However, the invention also applies to any other braking systems and in particular to braking systems in which there is a conventional hydraulic braking system and an auxiliary energy braking system.
In general, the invention can therefore be applied to a vehicle braking installation comprising an auxiliary energy service braking system and a muscle-powered braking system of the conventional type used for emergency braking.
A manually operated member such as a brake pedal actuates the service braking system or, should the latter fail or prove insufficient, the emergency braking.
Advantageously, the conventional braking system comprises a master cylinder fitted with at least one primary piston and is operated by the manual operating member (the brake pedal).
Furthermore, at least one safety valve allows the master cylinder to be isolated from the wheel brakes when the service braking system is operating normally. By contrast, when the braking force provided by the auxiliary energy braking system (the electrical braking system) is insufficient or when this braking system is defective, this safety valve allows the master cylinder to be coupled to at least one wheel brake.
In such systems, in order for the driver to have a feeling of braking when the external energy braking system is in operation, a braking feel simulator is provided in order to resist the forward movement of the manual operating member (the brake pedal), under auxiliary energy-powered service braking, with a reactive resistance force that reflects how braking is progressing.
This simulator commonly comprises a simulator piston sliding in a chamber. This piston is urged on one side (directly or indirectly) by the manual operating member and on another side by a means of applying a force which simulates the braking force.
When the auxiliary energy braking system is in operation, the master cylinder is therefore isolated from the wheel brakes and the liquid contained in the master cylinder is unable to flow back to these brakes. Thanks to the feel simulator, the manual operating member receives a force which resists the actuation of the manual operating member and simulates the braking force. In theory, this simulation system makes it possible to create a law governing the variation in the force to be applied to the manual operating member as a function of the travel. This gives the driver a feel similar to the feel he would obtain if the liquid pressure in the wheel brakes were the direct result of the pressure from the master cylinder and of the muscle power applied to the brake pedal.
In known simulation systems, the law governing the variation in the force to be applied to the manual operating member is determined chiefly by an elastic return means, generally formed by a spring, and the law cannot be modified simply and quickly, and this is a disadvantage. In addition, this elastic return means may have characteristics that vary over time or according to the type of vehicle.